U.S. patent number 6,674,937 [Application Number 10/044,804] was granted by the patent office on 2004-01-06 for optical wavelength routing circuits.
This patent grant is currently assigned to LC2I, Inc.. Invention is credited to Steven M. Blair, Larry L. Campbell.
United States Patent |
6,674,937 |
Blair , et al. |
January 6, 2004 |
Optical wavelength routing circuits
Abstract
The present invention comprises apparatuses and methods, for
routing optical communication signals over all DWDM wavelength
channels. Incoming wavelength channels are separated into a
plurality of sub-groups having a smaller optical bandwidth.
Wavelength channels within each sub-group are then acted upon
independently by a filter, or switch, tunable and operable over the
bandwidth of each sub-group. The invention may be embodied in a
plurality of dynamic wavelength routing circuits including
1.times.N and N.times.N circuits. The 1.times.N circuit embodiments
may be used when the filter free-spectral range is smaller than the
full DWDM bandwidth. These embodiments are less complicated than
the N.times.N circuit embodiments, but do not provide the same
routing bandwidth. The N.times.N circuit embodiments may be used to
route the full DWDM bandwidth and may be used for other routing
operations having large bandwidth demand.
Inventors: |
Blair; Steven M. (Salt Lake
City, UT), Campbell; Larry L. (Salt Lake City, UT) |
Assignee: |
LC2I, Inc. (Salt Lake City,
UT)
|
Family
ID: |
29738652 |
Appl.
No.: |
10/044,804 |
Filed: |
January 11, 2002 |
Current U.S.
Class: |
385/24; 372/20;
385/16; 385/22 |
Current CPC
Class: |
G02B
6/29383 (20130101); G02B 6/29395 (20130101); H04J
14/021 (20130101); H04J 14/0213 (20130101); H04Q
11/0005 (20130101); G02B 6/29317 (20130101); G02B
6/29356 (20130101); G02B 6/29358 (20130101); H04J
14/0209 (20130101); H04J 14/0212 (20130101); H04J
14/0217 (20130101); H04Q 2011/0009 (20130101); H04Q
2011/0075 (20130101) |
Current International
Class: |
G02B
6/34 (20060101); H04J 14/02 (20060101); H04Q
11/00 (20060101); G02B 006/28 () |
Field of
Search: |
;385/16-24,37 ;372/20,32
;359/124,130,127 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Palmer; Phan T. H.
Attorney, Agent or Firm: Coursey, Esq.; R. Steven Troutman
Sanders LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This patent application claims priority on and the benefit of the
filing date of U.S. provisional patent application No. 60/261,406
filed on Jan. 11, 2001.
Claims
What is claimed is:
1. An apparatus for optically routing wavelength channels over at
least a portion of the dense wavelength division multiplexed (DWDM)
bandwidth, said apparatus comprising: a first optical element
adapted to separate a plurality of input DWDM wavelength channels
into a plurality of sub-groups of DWDM wavelength channels, each
sub-group of DWDM wavelength channels having a bandwidth less than
the bandwidth of said plurality of input DWDM wavelength channels;
and, a plurality of tunable wavelength filtering elements, each
adapted to receive a respective sub-group of DWDM wavelength
channels from said first optical element and having a limited
tuning range selected for operation with he bandwidth of said
respective sub-group of DWDM wavelength channels.
2. The apparatus of claim 1, wherein a tunable wavelength filtering
element of said plurality of tunable wavelength filtering elements
comprises a tunable standing wave resonant cavity.
3. The apparatus of Claim 1, wherein a tunable wavelength filtering
element of said plurality of tunable wavelength filtering elements
comprises a tunable traveling wave resonant cavity.
4. The apparatus of claim 1, wherein a tunable wavelength filtering
element of said plurality of tunable wavelength filtering elements
comprises a tunable grating.
5. The apparatus of claim 1, wherein a tunable wavelength filtering
element of said plurality of tunable wavelength filtering elements
has a filter free-spectral range greater than the bandwidth of said
respective sub-group of DWDM wavelength channels receivable by said
tunable wavelength filtering element and less than the total
bandwidth across all input DWDM wavelength channels of said
plurality of input DWDM wavelength channels, and wherein said
limited tuning range of said tunable wavelength filtering element
is no greater than said filter free-spectral range.
6. The apparatus of claim 1, wherein a tunable wavelength filtering
element of said plurality of tunable wavelength filtering elements
has a filter free-spectral range greater than the total bandwidth
across all input DWDM channels of said plurality of input DWDM
wavelength channels, wherein said limited tuning range of said
tunable wavelength filtering element is not less than the bandwidth
of said respective sub-group of DWDM wavelength channels receivable
by said tunable wavelength filtering element, and wherein said
limited tuning range of said tunable wavelength filtering element
is less than said filter free-spectral range.
7. The apparatus of claim 1, wherein said first optical element and
said plurality of tunable wavelength filtering elements are
operable to perform tunable 1.times.N demultiplexing.
8. The apparatus of claim 1, wherein said first optical element and
said plurality of tunable wavelength filtering elements are
operable in reverse to perform tunable N.times.1 multiplexing.
9. The apparatus of claim 1, wherein said first, optical element
and at least one tunable wavelength filtering element of said
plurality of tunable wavelength filtering elements are operable to
tunably select a single input DWDM wavelength channel of said
plurality of input DWDM wavelength channels for placement of said
single input DWDM wavelength channel onto a drop output
channel.
10. The apparatus of claim 9, wherein said apparatus further
comprises a wavelength multiplexer operable to multiplex unselected
input DWDM wavelength channels of said plurality of input DWDM
wavelength channels onto a transmission output channel.
11. The apparatus of claim 1, wherein said first optical element
and said plurality of tunable wavelength filtering elements are
operable to tunably select respective input DWDM wavelength
channels of said plurality of input DWDM wavelength channels for
placement of said respective input DWDM wavelength channels onto a
drop output channel.
12. The apparatus of claim 11, wherein said apparatus further
comprises a wavelength multiplexer operable to multiplex said
respective input DWDM wavelength channels onto said drop output
channel.
13. The apparatus of claim 1, wherein at least one tunable
wavelength filtering element of said plurality of tunable
wavelength filtering elements is configurable to tune over one more
DWDM wavelength channel than the number of DWDM wavelength channels
present in said respective sub-group of DWDM wavelength channels
receivable by said at least one tunable wavelength filtering
element, and wherein said at least one tunable wavelength filtering
element is operable to allow all DWDM wavelength channels present
in said respective sub-group of DWDM wavelength channels to pass
when said at least one tunable wavelength filtering element is
configured to tune to said one more DWDM wavelength channel.
14. The apparatus of claim 1, wherein said first optical element
and said plurality of tunable wavelength filtering elements are
operable to perform tunable N.times.N wavelength switching.
15. An apparatus for optically routing wavelength channels over at
least a portion of the dense wavelength division multiplexed (DWDM)
bandwidth, said apparatus comprising: a plurality of tunable
wavelength filtering elements, each adapted to receive a respective
sub-group of DWDM wavelength channels of a plurality of DWDM
wavelength channels and having a limited tuning range selected for
operation with the bandwidth of said respective sub-group of DWDM
wavelength channels, the bandwidth of said respective sub- group of
DWDM wavelength channels being smaller than the total bandwidth of
said plurality of DWDM wavelength channels; wherein the filter
free-spectral range of at least one tunable wavelength filtering
element of said plurality of wavelength filtering elements is
greater than the total bandwidth across all DWDM wavelength
channels of said plurality of DWDM wavelength channels; and,
wherein said limited tuning range of said at least one tunable
wavelength filtering element is not less than the bandwidth of said
respective sub-group of DWDM wavelength channels receivable by said
at least one tunable wavelength filtering element, and wherein said
limited tuning range of said at least one tunable wavelength
filtering element is less than said filter free-spectral range.
16. The apparatus of claim 15, wherein a tunable wavelength
filtering element, of said plurality of tunable wavelength
filtering elements comprises a tunable standing wave resonant
cavity.
17. The apparatus of claim 15, wherein a tunable wavelength
filtering element of said plurality of tunable wavelength filtering
elements comprises a tunable traveling wave resonant cavity.
18. The apparatus of claim 15, wherein a tunable wavelength
filtering element of said plurality of tunable wavelength filtering
elements comprises a tunable grating.
19. The apparatus of claim 15, wherein said plurality of tunable
wavelength filtering elements are operable to perform tunable
1.times.N demultiplexing.
20. The apparatus of claim 15, wherein said plurality of tunable
wavelength filtering elements are operable in reverse to perform
tunable N.times.1 multiplexing.
21. The apparatus of claim 15, wherein said at least one tunable
wavelength filtering element of said plurality of tunable
wavelength filtering elements is operable to tunably select a
single DWDM wavelength channel of said plurality of DWDM wavelength
channels for placement of said single DWDM wavelength channel onto
a drop output channel.
22. The apparatus of claim 21, wherein said apparatus further
comprises a wavelength multiplexer operable to multiplex unselected
DWDM wavelength channels of said plurality of DWDM wavelength
channels onto a transmission output channel.
23. The apparatus of claim 15, wherein said plurality of tunable
wavelength filtering elements are operable to tunably select
respective DWDM wavelength channels of said plurality of DWDM
wavelength channels for placement of said respective DWDM
wavelength channels onto a drop output channel.
24. The apparatus of claim 23, wherein said apparatus further
comprises a wavelength multiplexer operable to multiplex said
respective DWDM wavelength channels onto said drop output
channel.
25. The apparatus of claim 15, wherein said plurality of tunable
wavelength filtering elements are operable to perform tunable
N.times.N wavelength switching.
26. The apparatus of claim 15, wherein at least one tunable
wavelength filtering element of said plurality of tunable
wavelength filtering elements is configurable to tune over one more
DWDM wavelength channel than the number of DWDM wavelength channels
present in said respective sub-group of DWDM wavelength channels
receivable by said at least one tunable wavelength filtering
element, and wherein said at least one tunable wavelength filtering
element is operable to allow all DWDM wavelength channels present
in said respective sub-group of DWDM wavelength channels to pass
when said at least one tunable wavelength filtering element is
configured to tune to said one more DWDM wavelength channel.
Description
FIELD OF THE INVENTION
The present invention relates, generally, to the field of apparatus
and methods for use in optical telecommunication networks and, in
its preferred embodiments, to the field of apparatus and methods
for dynamically routing wavelength channels in optical fiber DWDM
networks.
BACKGROUND OF THE INVENTION
In the modern age of information exchange, many companies depend
upon telecommunications networks to carry out their daily business
and often rely upon telecommunications providers to supply a fast
and reliable network with a very high bandwidth. For today's large
scale telecommunication applications, optical fiber dense
wavelength division multiplexed ("DWDM") networks appear to be a
very good first generation solution which address and/or meet such
requirements. The challenge, however, is to optimize the
capabilities of optical networks to create a second generation
network for use in the future.
The second generation of optical networks may use transparent
optical routing of numerous wavelength channels. While it is
desirable to route such wavelength channels entirely in the optical
domain using integrated optics technology, such routing presents a
number of technological challenges. One of the main challenges is
that such routing may require the use of wavelength-selective
filtering elements compatible with integrated optics.
Unfortunately, such wavelength-selective filtering elements are,
typically, not tunable over the entire wavelength range of
interest. For instance, with the advent of optical DWDM, the number
of wavelength channels requiring routing in a particular optical
fiber application may be on the order of one hundred to a thousand,
and the total optical bandwidth may range between ten nanometers
and hundreds of nanometers. Building a tunable wavelength filter
that can be tuned over such a wide DWDM bandwidth is difficult.
Another technological challenge stems from the need to route such
wavelength channels at high speeds on a microsecond or faster time
scale. These requirements place great demands on any technology,
and are difficult to achieve in concert.
Two important, "building-block" circuits for routing wavelength
channels in second generation optical and/or DWDM networks are
likely to be (1) the tunable add/drop, in which one of "N" incoming
wavelengths is dropped, and (2) the 1.times.N tunable wavelength
demultiplexer, in which "N" wavelengths on an input channel are
separated into "N" independent output channels, as shown in FIG. 1
(alternatively, the demultiplexer of FIG. 1 may be reconfigured to
drop multiple wavelengths onto a single output optical fiber by
using additional tunable wavelength filters, as depicted in FIG.
2). Due to reciprocity, these circuits may be used in reverse to
perform wavelength add and N.times.1 wavelength multiplexing,
respectively. In these circuits, 1 to "N" tunable wavelength
filters are used to selectively "drop", or direct, a selected
wavelength channel to an output optical fiber. Each such tunable
wavelength filter should be tunable over all wavelength
channels.
Another important circuit for routing wavelength channels in second
generation optical and/or DWDM networks is likely to be the
N.times.N wavelength routing switch circuit illustrated in FIG. 3.
This circuit takes "N" inputs, each with "N" wavelengths, and
routes one wavelength from each input to each output. Each output
receives all "N" wavelengths, with each wavelength originating from
a different input. As noted above, it is desirable for each of the
N.sup.2 wavelength filters to have the ability to independently
access all "N" wavelength channels.
Many of the prior art tunable filter technologies suitable for
dense integration on an optical chip cannot tune over the full DWDM
bandwidth, thus making implementation of the above-described
circuits (and a variety of other circuits) impractical. For
instance, arrayed waveguide gratings, acousto-optical and
electro-optical tunable filters, and Mach-Zehnder interferometer
techniques are so limited. Similarly, while being suitable for
dense integration, neither the traveling wave optical microcavity
filter nor longitudinal Bragg gratings can tune over the full DWDM
bandwidth.
The traveling wave optical microcavity filter, which includes a
tuned optical cavity, has resonances that allow the transfer of
specific wavelengths from an input optical channel to an output
optical channel. The tuned optical cavity supports whispering
gallery modes which behave very similarly to the longitudinal modes
of a linear Fabry-Perot type cavity. Fabry-Perot type resonators
may be implemented with optical fiber or integrated onto an optical
chip using reflective interfaces or longitudinal Bragg gratings.
The length of the cavity determines the resonance wavelengths;
which are the wavelengths that pass from the input channel through
the cavity to the output channel with high efficiency. These
wavelengths, or frequencies, given by ##EQU1##
are periodic, with the period being given by the cavity
free-spectral range ("FSR"), which is approximately ##EQU2##
where "c" is the speed of light, "n" is the effective index of the
cavity mode, and "L" is the round trip path length through the
cavity. In DWDM systems, it is generally beneficial to have
.DELTA..nu. be greater than the total optical bandwidth, which is
computed from the number of wavelength channels multiplied by the
channel spacing. By doing so, a single wavelength channel may be
operated upon without interference from other channels. Another
condition that must be met is to have the resonance frequency
passband, given by the expression ##EQU3##
where ".nu." is the resonance frequency and "Q" is the quality
factor of the cavity (i.e., which is related to the losses in the
cavity), be approximately equal to the wavelength channel spacing
.nu..sub.ch.
Additionally, it is desirable to have the ability to tune the
resonance frequency by one free-spectral range, so that all
wavelength channels may be operated upon by a single cavity. Such
tuning may be achieved by varying the index of refraction. To tune
over the entire free-spectral range by changing the index of
refraction requires that ##EQU4##
be achieved. For more general situations in which the tuning range
is less than the free-spectral range, the condition
.DELTA..nu./.nu.=.DELTA.n/n is still valid, where .DELTA..nu. now
represents the tuning range. For a number of tuning mechanisms,
such as the electro-optic effect and the thermo-optic effect, the
maximum achievable fractional index change, .DELTA.n/n, is of the
order 0.01, meaning that the maximum cavity free-spectral range
over which full tuning can be performed is
.DELTA..nu..apprxeq.0.01.nu..apprxeq.2 THz, which is much smaller
than the optical bandwidth of interest such as that made available,
for example, by optical fiber amplifiers, and therefore smaller
than the total bandwidth that may be used by high capacity DWDM
networks.
Alternatively, the length of the cavity could be changed by the
amount ##EQU5##
but again, large amounts of change, such as provided by the
piezoelectric effect, are difficult to achieve. Utilization of both
refractive index and cavity length changes may increase the tuning
by about a factor of two, but such an increase may still not be
enough to cover the desired wavelength range. However, it should be
noted that MEMS type devices with moving parts may achieve this
goal, but may be very difficult to stabilize to a specific
wavelength channel, as a positioning accuracy of ##EQU6##
must be attained, where ".delta.L" is the necessary positioning
accuracy. Therefore, these refractive index and length change
considerations make it very difficult for a single traveling wave
optical microcavity filter to be tunable over all wavelength
channels.
Similar to traveling wave optical microcavity filters and as noted
above, prior art longitudinal Bragg gratings, which may be
fabricated in an optical fiber or waveguide on an integrated
optical chip, also cannot tune over the full DWDM bandwidth. A
Bragg grating strongly reflects wavelengths that satisfy the
condition ##EQU7##
where ".LAMBDA." is the grating spacing, "n" is the refractive
index, and "i" is an integer. Bragg gratings are, typically,
fabricated such that the grating spacing is one-half the wavelength
(i.e., i=1), equal to the wavelength (i.e., i=2), or three-halves
the wavelength (i.e., i=3). The free-spectral range, FSR, can be
written as ##EQU8##
In the communications bands, for such grating spaces, the
free-spectral ranges would be approximately 200 THz, 100 THz, and
67 THz, respectively. Therefore, the free-spectral range of Bragg
grating type filters, typically, exceeds the DWDM spectrum.
The frequency tuning range, .DELTA..nu., of a Bragg grating may be
written in terms of a refractive index change or grating spacing
change ##EQU9##
where, again, the same limitations on refractive index and/or
cavity length change apply. Therefore, the total tuning range may
only be of the order of 2 THz, which is not sufficient to tune over
the full DWDM bandwidth.
Typical optical DWDM systems operate with a 100 GHz or, more
recently, a 50 GHz channel spacing. It is expected that such
systems may ultimately employ hundreds of wavelengths in each of
the "C" and "L" bands, as the width of each band is of the order of
10 THz. In order to implement filtering elements tunable over the
entire range of one such band, fractional changes of approximately
0.05 in filter parameters are needed. Using presently available
materials, a more practical fractional change of about 0.003, for
example, gives a tuning range of about 600 GHz, or about 6 (for 100
GHz spacing) or 12 (for 50 GHz spacing) wavelength channels.
Therefore, there exists in the industry, a need for optical
wavelength routing circuits having wavelength-selective filtering
elements compatible with integrated optics and operable at high
speeds which are tunable over all DWDM channels, and which address
these and other related, and unrelated, problems.
SUMMARY OF THE INVENTION
Briefly described, the present invention comprises apparatus and
methods, tunable and operable over all DWDM wavelength channels,
for optically routing such channels. According to the present
invention, incoming wavelength channels are separated into a
plurality of sub-groups having a smaller optical bandwidth.
Wavelength channels within each sub-group are then acted upon
independently by a filter, or switch, which is tunable and operable
over the reduced bandwidth of each sub-group. Where necessary, the
sub-groups are then recombined to form a desired output composite
channel.
In the preferred embodiments described herein, the present
invention is embodied in two classes of dynamic wavelength routing
circuits, including, for example and not limitation, 1.times.N and
N.times.N circuits. In the first class of circuits, the filter
free-spectral range (and, therefore, the necessary tuning range) is
a fraction of the full DWDM bandwidth, and is represented as the
bandwidth of a sub-group of wavelength channels. These circuits
require that the wavelengths be divided into sub-bands by a DWDM
demultiplexer and are more suitable for filters whose free-spectral
range is typically less than the DWDM bandwidth (e.g., resonant
cavities), such that tuning within each sub-group may be performed
with filters having a single filter design. In the second class of
circuits, the circuits include filters having a free-spectral range
greater than the full DWDM bandwidth, but still have a limited
tuning range (e.g., the filters include Bragg gratings and resonant
cavities of small size). The filters of circuits in the second
class must tune over different wavelength ranges to cover the
entire DWDM bandwidth. Note that most types of circuits devised for
the first class may be implemented by the second class of circuits,
and vice versa. Circuits which represent a hybrid between the two
classes are also described herein, and may provide additional
flexibility not offered by circuits of the first or second classes
alone.
It should be understood that while the present invention is
described herein in the form of particular illustrative circuits,
the present invention has applicability to a plurality of other
circuits as well. Also, as described herein, practical filters may
be tunable, for example, over six to twelve wavelength channels,
while there may be a hundred or more channels, for example, which
require routing. Therefore, use of the present invention for
routing the entire number of possible channels may require division
of the DWDM bandwidth into ten to twenty sub-groups of twelve to
six wavelengths each. However, for simplicity of description and
illustration, the circuits herein are described herein with
reference to a greatly reduced number of wavelength channels. Thus,
it is important to note that the present invention is not limited
by the artificial constraints imposed for purposes of the
description herein.
Accordingly, it is an object of the present invention to provide an
apparatus and/or method for separating an incoming optical waveform
into a plurality of sub-groups of wavelength channels having a
smaller optical bandwidth than the incoming optical waveform.
Another object of the present invention to provide an apparatus
and/or method for separating an incoming optical waveform into a
plurality of sub-groups of wavelength channels having a smaller
optical bandwidth than the incoming optical waveform and for
recombining the sub-groups to form a desired output channel.
Still another object of the present invention to provide an
apparatus and/or method for separating an incoming optical waveform
into a plurality of sub-groups of wavelength channels having a
smaller optical bandwidth than the incoming optical waveform and
for tuning the channels of each sub-group.
Still another object of the present invention to provide an
apparatus and/or method for cumulatively tuning the entire DWDM
bandwidth by tuning a plurality of sub-groups of wavelength
channels having a smaller optical bandwidth.
Other objects, features, and advantages of the present invention
will become apparent upon reading and understanding the present
specification when taken in conjunction with the appended
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram representation of a tunable wavelength
add/drop circuit and a demultiplexer in accordance with prior
art.
FIG. 2 is a block diagram representation of a demultiplexer
configured to drop multiple wavelengths onto a single output
optical fiber in accordance with prior art.
FIG. 3 is a block diagram representation of an N.times.N tunable
wavelength routing switch in accordance with prior art.
FIG. 4 is a block diagram representation of tunable filter elements
based upon resonant optical cavities in accordance with prior
art.
FIG. 5 is a block diagram representation of a tunable 1.times.N
wavelength demultiplexer using wavelength sub-groups, according to
an exemplary preferred embodiment of the present invention.
FIG. 6 is a block diagram representation of a tunable wavelength
switch using wavelength sub-groups, according to an exemplary
preferred embodiment of the present invention.
FIG. 7 is an illustration of a reduced tuning range, according to
an exemplary preferred embodiment of the present invention.
FIG. 8 is a block diagram representation of a tunable wavelength
switch using simplified wavelength grouping, according to an
exemplary preferred embodiment of the present invention.
FIG. 9 is a block diagram representation of a tunable
multi-wavelength add/drop circuit using wavelength grouping,
according to an exemplary preferred embodiment of the present
invention.
FIG. 10 is a block diagram representation of an N.times.N
wavelength switch, according to an exemplary preferred embodiment
of the present invention.
FIG. 11 is a pictorial representation of a tuning range for
resonant filters with large free-spectral range, according to an
exemplary preferred embodiment of the present invention.
FIG. 12 is a block diagram representation of a tunable single
wavelength add/drop circuit using wavelength grouping, according to
an exemplary preferred embodiment of the present invention.
FIG. 13 is a pictorial representation of a blank wavelength channel
within a wavelength sub-group for making a filter transparent,
according to an exemplary preferred embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings, in which like numerals represent
like components or steps throughout the several views, FIG. 5
displays an exemplary circuit of the first class of circuits (i.e.,
which includes circuits having a free-spectral range that is a
fraction of the full DWDM bandwidth), including a 1.times.N
demultiplexer, which is employed in or with a plurality of other
circuits of the first class of circuits in a first set of preferred
embodiments of the present invention. In the embodiments of the
first set, a group of "N" incoming wavelength channels 501 are
split into "M" sub-groups 503-506 using a wavelength demultiplexer
501 or similar wavelength filter (which is typically fixed, but may
itself be tunable), with N/M wavelengths per sub-group. Tunable
filters 507-510 then perform wavelength routing within each
sub-group.
As shown in FIG. 5, a single input channel 502 (e.g., optical fiber
or integrated optical waveguide), preferably, carries 16
wavelengths. These wavelengths are broken into four sub-groups
503-506 of four wavelengths each, with each sub-group carried by a
separate path. Within each path there are four tunable wavelength
filters 507-510 (i.e., where different subscripts indicate
different sub-group filters), each of which can drop any one of the
four wavelengths to an output channel 511. Each filter has a
free-spectral range given by ##EQU10##
where .nu..sub.ch is the DWDM channel spacing. Thus, the circuit
reduces the necessary free-spectral range by the factor "M" and,
therefore, reduces the tuning requirements (i.e., such as the
necessary refractive index or length change) by a factor of "M" as
well.
The circuit of FIG. 5 may be used to implement an N.times.N
wavelength switch using the 1.times.N demultiplexer as a building
block, as shown in FIGS. 6 and 8. As described above, "N"
wavelengths from each input are separated into N/M sub-groups by
demultiplexer 601, where the sub-groups for each input are not
necessarily the same. Each wavelength within each sub-group is
routed to one of "M" outputs, where there are a total of "N"
outputs (e.g., 604-607). Such circuits allow considerable
flexibility in a wavelength routing process, in that arbitrary
routing is possible given a fixed set of constraints.
In the circuit of FIG. 6, each input optical fiber carries four
wavelengths for simplicity of description and illustration. The
wavelengths are split into two sub-groups 602, 603 of two
wavelengths at the input to the routing matrix 608 (i.e., where the
squares in matrix 608 of FIG. 6 represent fixed or tunable filters
610). In order to reduce the free-spectral range of each filter
610, the wavelengths must be split such that no two wavelengths
within a sub-group are separated by an integer multiple of the
filter free-spectral range. All such possible (i.e., four of the
six total) arrangements using four wavelengths are employed in the
circuit of FIG. 6. These arrangements are displayed in more detail
in FIG. 7, which shows the four possible choices of wavelength
sub-groups obtained by breaking four wavelengths into groups of two
wavelengths. All four of these groupings are utilized in the
circuit of FIG. 6, while only two are used in the circuit of FIG.
8. In general, one would choose "M" wavelengths within a sub-group,
so that the filter free-spectral (or tuning) range would be the
product of "M" and the channel spacing. Within each sub-group, one
cannot have wavelengths separated by a multiple of the
free-spectral range. Otherwise, more than one wavelength could be
filtered at a time since filter tuning is periodic with the period
being given by the free-spectral range. In order to obtain full
coverage of the input channel spectrum, one must have N/M
sub-groups.
The circuit of FIG. 8 reduces the requirements on the DWDM
demultiplexer at the inputs. In this circuit, the input wavelength
band is split sequentially into N/M sub-groups, and the sub-groups
are the same across all inputs (801-804). The circuit of FIG. 8
allows the use of wavelength demultiplexers 805-808 with simplified
passbands (i.e., where the width of the passband is approximately
equal to the product of the number of wavelengths in the sub-group
and the channel spacing) to be used for each sub-group. In the
circuit of FIG. 8, the wavelength demultiplexer at each input
channel needs only have two passbands, one to pull out wavelengths
1 and 2, and one to pull out wavelengths 3 and 4. A demultiplexing
filter, acceptable in accordance with the circuit of FIG. 8,
includes Bragg gratings, resonant cavity type filters, phased array
filters, or other similar filters.
The multi-wavelength add/drop circuit of FIG. 9 represents another
circuit in accordance with the first set of preferred embodiments
of the present invention. As in the other circuits according to the
first set of preferred embodiments, the filter free-spectral range
is a fraction of the full DWDM bandwidth. In this circuit, sixteen
wavelengths on an input fiber 901 are broken into two sub-groups
902, 903 along two separate paths. The wavelengths chosen for each
sub-group are subject to the restrictions described above. The
wavelengths that are not filtered out (i.e., not dropped) are
combined by a passive wavelength multiplexer 904 to continue on the
transmission channel 907. The dropped wavelengths are also combined
by a wavelength multiplexer 906 onto a single drop channel 905.
Because the wavelength groupings remain fixed throughout the entire
circuit, the wavelength demultiplexer 903 and multiplexers 904, 906
have identical passbands. In addition, this circuit may also be
used to add wavelength channels by routing, or placing, those
channels on the drop output. The filters 610 are tuned to pick-off
any one of the eight wavelength channels in their respective
sub-groups.
The embodiments of the first set of preferred embodiments described
above employ wavelength demultiplexers to reduce the number of
frequency channels incident on each tunable filter 610, which
therefore reduces the necessary free-spectral range. In those
embodiments, the filter tuning occurs over the entire free-spectral
range of the sub-group. In order to accommodate the reduced
free-spectral range, no two frequency channels within a sub-group
can be separated by an integer multiple of the free-spectral range,
so that the filter drops only one wavelength at a time. In those
embodiments, all of the filters are substantially identical.
Further, every circuit has an analog suitable for large
free-spectral range filters, but the wavelength patterns within
each sub-group may differ.
In a second set of preferred embodiments of the present invention,
the circuits eliminate the need for wavelength demultiplexers at
the inputs, but include filters that are all different (i.e., in
the sense that their bare resonant frequencies are different) and
have larger free-spectral ranges (i.e., larger than those of the
circuits of the first set of preferred embodiments) that are
greater than the bandwidth of all wavelength channels. These
embodiments are more appropriate for grating-based tunable filters
and small resonant cavities. Also, in these embodiments, the
wavelengths must be grouped in a sequential manner. For brevity,
only one such embodiment is described below, but it is understood
that other embodiments are within the scope of the present
invention.
FIG. 10 displays an N.times.N wavelength routing switch circuit
1000, in accordance with the second set of preferred embodiments,
which comprises a plurality of filters (or switches) 1010 having a
limited tuning range (i.e., less than the free-spectral range in
the case of resonant filters). The routing switch circuit 1000, for
purposes of simplifying the description, has only four wavelengths
per input channel 1030. As seen in FIG. 10, the routing switch
circuit 1000 includes two different types of filters: a first
plurality of filters 1010 that are tunable over wavelengths 1 and
2; and, a second plurality of filters 1020 that are tunable over
wavelengths 3 and 4. The difference in the filters 1010, 1020 may
be created through the use of a difference in cavity length for
resonant filters or a difference in grating spacing for Bragg
grating filters. The operation of the resonant filters 1010, 1020
of the N.times.N wavelength routing switch circuit 1000 (i.e.,
those with large free-spectral range and small tuning range) is
illustrated by FIG. 11. Operation with grating filters is
similar.
In many optical wavelength routing applications, it is desirable to
drop one of "N" wavelengths from an input channel onto a single
output channel. With prior art devices, a tunable filter having a
tuning range over the entire DWDM bandwidth would be required to
implement the dropping of the wavelength. However, using the
present invention, the dropping of the wavelength is accomplished
by splitting the input wavelengths into N/M sub-bands (or
sub-groups), as illustrated for sixteen input wavelengths and four
sub-bands (or sub-groups) with the hybrid circuit 1200 of FIG. 12.
The hybrid circuit 1200 utilizes DWDM multiplexers 1202, 1203 and
demultiplexer 1201 in order to perform wavelength grouping and
utilizes filters 1204-1207 with large free-spectral range and
different tuning ranges.
Since one wavelength from only one of the four sub-bands (or
sub-groups) is to be dropped, a method for making each filter
1204-1207 completely transparent must be utilized. According to
such a method, each filter 1204-1207 is allowed to tune over a
frequency range given by the product of M+1 and the channel
spacing. Since only "M" wavelengths are present within each
sub-band (or sub-group), the extra tuning location is therefore
empty, and tuning to this position will allow all "M" wavelengths
in the sub-band to pass without being dropped, as shown in FIG. 13.
In order to reduce operating power requirements, it is desirable to
make the location of the empty tuning slot equal to the passband
location of the filter with no physical parameter change. By doing
so, only the filter 1204-1207 that need be tuned is the one
selecting the desired wavelength from the appropriate sub-band and,
hence, less operating power is required.
Whereas this invention has been described in detail with particular
reference to its preferred embodiments, it is understood that
variations and modifications can be effected within the spirit and
scope of the invention, as described herein before and as defined
in the appended claims. The corresponding structures, materials,
acts, and equivalents of all means plus function elements, if any,
in the claims below are intended to include any structure,
material, or acts for performing the functions in combination with
other claimed elements as specifically claimed.
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